This invention relates to the field of communications systems and, in particular, to a method and apparatus for utilizing transmission bandwidth more efficiently in Dense Wavelength Division Multiplexed systems.
To meet today""s demand for high-speed cost-effective communications, optical transmission systems having increased data capacity are highly desirable. One approach used in modem high-capacity transmission systems to increase the aggregate data-rate of transmission systems is to use a technique called dense wavelength division multiplexing (DWDM). In DWDM, an optical transmission link is divided into a plurality of channels with each channel having its own center frequency. Data transmitted on a particular channel is then effected by modulating the optical carrier at the center frequency of that channel. At the receiver, a band-pass filter tuned to the center frequency of the channel is used for detecting and demodulating the transmitted signal. By combining a plurality of channels in this manner, the aggregate data capacity of the optical link is increased. For example, using this technique, optical transmission systems with an aggregate data-handling capacity of 1 terabit per second have been demonstrated. See H. Onaka, et al. xe2x80x9c1.1 Tb/s WDM transmission over 150 km 1.3 mm zero-dispersion single mode fiber,xe2x80x9d Proc. OFC ""96, PD19, 1996; A. H. Gnauck, et al, xe2x80x9cOne terabit/s transmission experiment,xe2x80x9d Proc. OFC ""96, PD20, 1996; T. Morioka, et al. xe2x80x9c100 Gbit/sxc3x9710 channel OTDM/WDM transmission using a single supercontinuum WDM source,xe2x80x9d Proc. OFC ""96, PD21, 1996; Y. Yano, et al. xe2x80x9c2.6 Tb/s WDM transmission experiment using optical duobinary coding,xe2x80x9d Proc. ECOC ""96, ThB3.1, 1996. A limitation in increasing the aggregate data-handling capacity of optical transmission systems is the amount of separation required between adjacent channels sufficient to reduce cross-channel interference to acceptable levels. Channel separations in the range of 100 GHz are commonly used to achieve sufficient separation.
A drawback of prior art optical transmission systems is that the aggregate data rate presently achievable, 1 Tb/s, is still orders of magnitude below the total capacity of optical fiber. In other words, the spectral efficiency (defined as the ratio between the aggregate bit rate transmitted over the optical link and the total optical bandwidth) of prior art systems is not maximized for several reasons. First, the need to maintain channel separation of 100 GHz to reduce interference between channels reduces the number of channels that can be multiplexed on the optical link. As a result, the aggregate bit rate of the optical link is limited thereby reducing the spectral efficiency of the transmission system. Also, because dispersion and nonlinearities in the optical transmission link limits the modulation bandwidth, and thus the bit-rate of any particular signal channel, the spectral efficiency of the system is decreased. Spectrally efficient signaling techniques, such as duobinary signaling, have been investigated in an attempt to reduce the spectral bandwidth required for each particular channel so that more channels can be supported by an optical link. See A. Lender, xe2x80x9cCorrelative digital communication techniques,xe2x80x9d IEEE Trans. Commun. Technol., vol. COM-12, p. 128, 1964; X. Gu and L. C. Blank, xe2x80x9c10 Gbit/s unrepeatered three-level optical transmission over 100 km of standard fiber,xe2x80x9d Electron. Lett., vol. 29, p. 2209, 1993; A. J. Price and N. Le Mercier, xe2x80x9cReduced bandwidth optical digital intensity modulation with improved chromatic dispersion tolerance,xe2x80x9d Electron. Lett., vol. 31, p. 58, 1995; D. Penninckx, et al, xe2x80x9cRelation between spectrum bandwidth and the effects of chromatic dispersion in optical transmissions,xe2x80x9d Electron. Lett., vol. 32, p. 1023, 1996.
In duobinary signaling the required spectral bandwidth for a channel is reduced by manipulating the phase of the output data symbols transmitted over that channel. In duobinary, the data to be output consists of a combination of zeros and ones. The phase of the output data symbols are selected as follows: 1""s in the input data stream that are separated by an even number of 0""s have an identical phase in the duobinary signal output while 1""s that are separated by an odd number of 0""s have an opposite phase to that of the previously output 1. For example, the input data sequence {1, 1, 0, 1, 0, 0, 1, 1} is converted to a duobinary signal output of {1, 1, 0, xe2x88x921, 0, 0, xe2x88x921, xe2x88x921} where xe2x88x921 denotes a data bit having an opposite phase of a 1 data bit. Although duobinary signally does increase spectrally efficiency of the transmission system by narrowing the spectral bandwidth required for a channel, it is desirable to provide other signaling techniques that further increase the spectral efficiency of optical transmission systems.
The present invention is directed at overcoming the shortcomings of the prior art. The present invention is directed to a method and apparatus for increasing the spectral efficiency of optical DWDM transmission systems by increasing the amount of data that can be transmitted through a channel without increasing the spectral bandwidth of the channel. The method of the present invention includes the step of outputting a symbol from a sequence of symbols to be output. Next, a spectral sum is calculated by summing the numerical values of all of the symbols that have been previously output. Next, a first sum is calculated by adding the numerical value of the next symbol to be output with a positive phase to the spectral sum calculated previously. Next, a second sum is calculated by adding the numerical value of the next symbol to be output with a negative phase to the spectral sum. The next symbol will be output with a positive phase if the magnitude of the first sum is less than the magnitude of the second sum. Next, the next symbol will be output with a negative phase if the magnitude of the first sum is greater than the magnitude of the second sum. If the magnitude of the first sum equals the magnitude of the second sum, the next symbol is output with either a positive phase or a negative phase. By manipulating the phase of the output symbol in such a manner, the spectral bandwidth of the output signal is band-limited to the Nyquist frequency even for quaternary signaling thus providing a two fold increase in spectral efficiency over duobinary.
In an alternative embodiment of the present invention, the set of all symbols that can be output is divided into a number of groups. If the next symbol to be output is, for example, from the first group, then the phase of the symbol is selected based on the number of symbols that were output from other groups since the last symbol that was output from the first group. If that number is odd, then the next symbol is output with the opposite phase from that of the last symbol output from the first group. If the number is even, then the next symbol is output with the same phase as that of the last symbol output from the first group. Selecting the phase in such a manner increases the spectral efficiency of the output signal as compared to duobinary.
In another alternative embodiment of the present invention, the set of all symbols that can be output is divided into a number of groups. If, for example, the next symbol to be output is from the first group, then the phase of the symbol will be the opposite of the phase of the last symbol output from the first group if there were any symbols output from other groups since the last symbol output from the first group. If no intervening symbols exist, then the phase of the next output symbol will be the same as the last symbol output from the first group. Selecting the phase in such a manner produces an output signal having a narrower spectral bandwidth and therefore a more spectrally efficient signal.
Other objects and features of the present invention will become apparent from the following detailed description, considered in conjunction with the accompanying drawing figures. It is to be understood, however, that the drawings, which are not to scale, are designed solely for the purpose of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.